The present invention relates to apparatus and method for the control of current in inductive loads and more particularly, to the use of a pulse-width-modulated (PWM) signal to effect accurate control over the current waveform.
The control of current through an inductive load is, for example, often necessary for the control of magnetic flux or heat produced.
An example of the need for accurate current control relates to proportional solenoid control and the need accurately to control the output force. In operation, the current in the solenoid must be continuously variable to provide a continuously variable force and smooth changes from one force level to another.
Such solenoids are used in hydraulic valves to provide continuously variable position control or pressure control. If no position or pressure feedback is used, the overall accuracy of the valve depends on the accuracy of control of the force produced by the proportional solenoid.
A continuously variable drive current can be produced by driving a suitable linear amplifier with the control signal. However, this involves a high energy dissipation in the amplifier. It has, therefore, become known to use a PWM (pulse width modulated) signal to drive the solenoid.
The energy dissipated by a PWM drive is substantially reduced because the power switching elements are either fully on or fully off. To control current with such a drive, the period of time during which the drive is on is adjusted relative to the period of time during which the drive is off in such a way that the average current is proportional to the control signal. The ratio between the on time and the off time is often referred to as the mark/space ratio.
In the case of an inductive load, the rate of change of current is limited. This means that the mark/space ratio must be adjusted to bring the current quickly to the desired amplitude then adjusted to give the desired average current.
This basic arrangement suffers from the disadvantage that the mean current to the solenoid depends not only on the amplitude of the control signal but also on the magnitude of the voltage of the power supply which is being switched by the modulator. This disadvantage is usually overcome by providing a solenoid current sensor the output of which is fed back to the modulator. This feedback signal allows the modulator to change the mark/space ratio to bring the current in the load to the correct level.
It has also been found that a fixed modulation ratio may result in poor response because of hysteresis effects in the solenoid. This can be overcome by superimposing a small "dither" signal on the control signal. This is usually achieved by providing a dither oscillator, which produces a small low-frequency signal which is added to the control signal before the control signal is fed to the modulator. It is preferable for the dither frequency to be synchronized with the modulator frequency but this may not readily be achieved if the PWM frequency changes significantly.
A further problem with the pulse modulator technique relates to modulators which operate at a fixed current ripple amplitude; this results in the current ripple amplitude being large in relation to the average current at low current levels.
The techniques discussed so far have all been essentially analogue in nature. In particular, it has been assumed that the control signal is an analogue signal. In many situations, however, the system in which the solenoid is included is largely digital in nature. The control signal may therefore be essentially digital, being generated by a microprocessor or some other similar digital circuitry. The problem therefore arises of driving a proportional solenoid from a digital control signal.
One possible solution is to arrange for the control source to generate a PWM signal directly, and feed that to a suitable drive circuit which feeds the solenoid. That arrangement suffers from the disadvantage of the simple pulse width modulator circuit mentioned above--that the average current to the solenoid varies with the power supply voltage.
Another possible solution is to generate the control signal as a multi-bit signal which is then decoded by a digital-to-analogue converter which drives an analogue circuit as discussed above. This allows current feedback but the problem of dither signal synchronisation and the problem cf the current ripple amplitude being large relative to the current at low current levels remains. Further, the system requires the analogue modulator and dither oscillator, which are relatively costly, to be retained.
The object of the invention is to provide a digital PWM current controller in which the above problems are alleviated or overcome. In many situations the current controller may be used in a system which itself is digitally controlled and the current demand signal originates as a digital signal but the controller may also be advantageously employed if the demand signals are analogue.